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Osprey
Pandion haliaetus
Order
ACCIPITRIFORMES
– Family
PANDIONIDAE
Authors: Poole, Alan F., Rob O. Bierregaard, and Mark S. Martell

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Introduction

Osprey, adult male at feeding perch
Adult male Osprey, with trout; Utah, June.
Fig. 1. Distribution of Ospreys breeding in North America.

Arguably North America’s best-studied bird of prey, and certainly one of its most admired, the Osprey is the continent’s only raptor that eats almost exclusively live fish. Despite this restriction, Ospreys have colonized a broad array of habitats. One finds their prominent stick nests from mangrove islets of the Florida Keys to coastal rivers of Labrador, from Alaskan lakes to Montana reservoirs, from New England salt marshes to the saline lagoons of Baja, Mexico, and from Carolina cypress swamps to the foggy redwood coasts of California. All but southernmost populations are migratory, vacating their breeding grounds in late summer for rain-forest rivers and fish-rich seacoasts and lakes of Central and South America, returning north each spring as waters warm and fish become accessible. An Osprey nesting in central Quebec and wintering in southern Brazil might fly more than 200,000 kilometers in migration during its 15-to 20-year lifetime. Clearly this is a mobile, adaptable creature, familiar with vast distances and a shifting complex of weather, prey, and habitat.

Ospreys dive feet first for their prey, accessing only about the top meter of water, so they are restricted to surface-schooling fish and to those in shallows—the latter generally most abundant and available. Thus North America’s Ospreys tend to breed most densely where shallow waters abound: Long Island Sound, Chesapeake Bay, and Florida Bay along the Atlantic coast; Baja Mexico’s Pacific coast; Georgian Bay in the Great Lakes; and several large reservoirs and lakes in western states. In many of these regions, as in others, artificial nest sites have helped breeders enormously in recent decades. Historically Ospreys built their nests atop trees, rocky cliffs and promontories, and—on a few islands free of mammalian predators—even on the ground. While some continue to use such natural sites, many have shifted to artificial sites, an astonishing array of them: channel markers in harbors and along busy waterways; towers for radio, cell-phone, and utility lines; and hundreds of nesting poles erected just for this species. This shift has been dramatic in many regions, with 90–95% of pairs choosing artificial sites; predation, loss of trees, and development of shorelines have been driving forces behind the change.

North American Ospreys gained increased recognition during the 1950s–1970s because populations in several key regions crashed. About 90% of the pairs nesting along the coast between New York City and Boston, for example, disappeared during this period; Chesapeake Bay lost about half its breeders; Great Lakes populations also suffered major declines. Studies showed high levels of contaminants (especially DDT and its derivatives) in eggs, severe eggshell-thinning, and poor hatching success. Mortality of adults may have contributed to the decline. Osprey studies provided key evidence in court to help block continued use of persistent pesticides, and Osprey populations recovered rapidly thereafter. Although small pockets of contamination remain, apparently mostly on wintering grounds, by the year 2000 many U.S. and Canadian populations were approaching historical numbers, boosted by a cleaner environment, by increasingly available artificial nest sites, and by this bird’s ability to tolerate human activity near its nests. Phoenix-like, the Osprey has arisen from the ashes of its own demise, a survivor, even a backyard bird in some areas; little wonder the species has become such a powerful totem for conservationists.

Concern for populations impacted by pesticides spawned a multitude of studies on various aspects of Osprey life history during the 1970s and 1980s, and these have continued almost undiminished since then—the product of Osprey allure, ease of study (highly visible nesting and hunting, and often accessible nests), and broad distribution. Bent (1937), Palmer (1988), and Poole (1989a) summarized North American Osprey life history through the late 1980s; Cramp and Simmons (1980) did the same for Ospreys breeding in the Palearctic. Although this account, by necessity, leans on these earlier works, we try to emphasize studies since then. Areas in which new ground has been broken are: analyses of migration based on satellite telemetry (Martell et al. 2001), opening an interesting window on Osprey movements outside the breeding season; behavior at the nest, based on studies in the UK and Corsica, but applicable to North American populations (Birkhead and Lessells 1988, Bretagnolle and Thibault 1993); molecular-genetic studies, helping to solidify Osprey taxonomy (Seibold and Helbig 1995); foraging and development of dietary preferences in the postfledging period (Edwards 1988, 1989); assessment of contaminants in western populations (Henny et al. 1991; Elliott et al. 1994, 2000, 2001); status of Great Lakes populations (Ewins 1995, 1996, 1997; Ewins et al. 1995, 1999); sibling aggression among nestlings (Forbes 1991); colonial nesting (Hagan and Walters 1990); lifetime reproduction (Postupalsky 1989b); and growth of nestlings (Steidl and Griffin 1991, Schaadt and Bird 1993). Status, ecology, and population dynamics of Palearctic Ospreys were detailed recently in Vogelwelt 116 and 122.